1. Field of Invention
The present invention relates to positioning mechanisms and more particularly to a highly precise closed-loop feedback control positioning stage.
2. Related Art
Conventionally, lead screws, gears, belts, bearings, and linear rails are employed in a mechanical feeding system. Such components are low in precision due to tolerance or low in positioning accuracy due to assembly error. Moreover, a stage is not stable due to friction between components. As such, above drawbacks must be substantially eliminated if it is desired to apply the mechanical feeding systems in nano based applications. Unfortunately, these drawbacks are still bottleneck yet to be solved. As a result, ultra-precise positioning systems are high in the manufacturing cost.
Conventionally, flexure structure is employed in positioning systems. In detail, stage is made flexible (i.e., flexure stage). A piezoelectric actuator is used as drive of the stage. A highly precise positioning of the stage is thus achieved by flexibly deforming the actuator. Flexure structure is thus used as a replacement of earlier revolute pair, prismatic pair, and spherical pair. By utilizing this, assembly error, component friction, stick loss, temperature abnormal rise, and low rigidity are substantially eliminated. Moreover, micro-machining can be facilitated when a stage is made even smaller as time evolves.
Such piezoelectric stages are widely employed in nano based detections and nano based manufacturing processes. However, piezoelectric stages have drawbacks such as creep and hysteresis, resulting in a non-linear effect as shown in FIG. 1 where position versus time for a curve of such piezoelectric stage is plotted. The non-linear effect in turn adversely affects positioning of the piezoelectric stage. Typically, the non-linear effect can be eliminated by closed-loop feedback control. Flexure stage is advantageous for having no gap and no assembly error. But there is error caused by interference between two orthogonal axes on the same plane. Such error also is required to be eliminated by closed-loop feedback control.
U.S. Pat. No. 6,555,829 discloses a high precision flexure stage having a closed-loop feedback control mechanism as shown in FIG. 2. Two bimorph piezoelectric actuators 10 and 20, four flexure bearings 11, 12, 13, and 14, and four flexure bearings 21, 22, 23, and 24 are defined in two channels respectively with the actuator 10 and 20 located therein. Transducers 15 and 25 monitor X and Y movement respectively of a stage in response to movements of actuators 10 and 20. The output signals of transducers 15 and 25 are monitored by a processor 30 which also controls the bending of actuators 10 and 20. The above components form a closed-loop feedback control system which enables processor 30 to precisely position an object mounted in the stage. The positioning precision of the transducer is proportional to that of the stage. Prior transducers are capacitive position sensors or laser interferometers. For the capacitive position sensors, distance between the sensor and an object to be tested is in the range of several micrometers to several hundred micrometers. In other words, precision requirement is very high. Also, advanced alignment equipment is required for correction. Further, its circuitry is complicated. For laser interferometer, it is bulky and is very high in price. As a result, the prior closed-loop feedback control positioning stage is difficult of design and thus high in the manufacturing cost. Thus, it is desirable to provide a novel closed-loop feedback control positioning stage in order to overcome the above drawbacks of prior art.